Preparation And In Vitro Chemo-mechanical Behavor Of Low-defect Wollastonite Coatings

Wollastonite coatings deposited on the Ti alloys substrate by plasma spraying possess high bonding strength and excellent bioactivity and biocompatibility, indicating they can be candidates for bone graft in human body. However, wollastonite coatings may be dissolved and degraded to induce the growth of bone tissues when implanted into the organism. To avoid the premature dissolution under physiological load bearing conditions, the coatings should possess high crystallinity, excellent mechanical properties and low defect. The effect of substrate temperature on the morphology of the splat and the crystallinity, microstructure, phase composition, mechanical properties and dissolution behavior of wollastonite coatings were investigated. In addition, the effect of interaction of chemo-mechanics on the microstructure and dissolution behavior of wollastonite coatings were investigated, which is more important to guarantee the service safety of coatings.The results show that when preheating temperature of the substrate is room temperature, the splashing occurs during droplet flattening which results in the formation of the splats with an irregular morphology. However, when the preheating temperature exceeds 115℃, the regular disk type splats are formed. The mean crack spacing decreases with the substrate temperature. An obvious increment of crystallinity, in the contrary, a decrement of porosity and solubility were found with the substrate temperature. The hardness and elastic modulus of coatings increased with the substrate surface temperature up to 400℃firstly, and then a decrement was observed with the temperature further increasing to 600℃. The hardness and elastic modulus oriented parallel to the spraying direction are less than those oriented perpendicular to the spraying direction. The ratio of hardness to elastic modulus shows a slightly decrement tendency with the substrate temperature, meaning the tougher deposit.It is revealed that tensile stress promotes the dissolution of wollastonite coatings, thus promoting the formation of apatite on the surface of coatings. However, compressive stress suppresses the dissolution of top deposits, and then inhibiting the formation of apatite on the surface of coatings. The dissolution of the internal parts of wollastonite coatings is promoted by SBF penetrated, owing to the delamination of coatings being attributed to compressive stress.